A common problem in switching circuits for AC signals is switching transients which occur as a result of changes in the average DC current level at the switch output. Such transients can cause output errors in precision applications and, in particular, can cause artifacts (i.e. undesired images) when the switch is being used as part of a display control apparatus.
One application where this problem arises is in phased array ultrasonic imaging systems where an image is generated of, for example, portions of a patient's body through which an ultrasonic beam is passing at successive time intervals. In such a system, a number of elements of a transducer array are utilized to perform a particular scan at a particular angle and/or to focus the ultrasonic beam. For example, with a transducer array having 128 or 256 transducer elements, as few as two such elements might be utilized for a given window. The number of transducer elements utilized may be expanded as a function of depth or for different applications, with all transducers in the array being sometimes utilized.
Since in such systems, the distance from each of the transducer elements to the point in the body being scanned varies with scan depth, the received ultrasound signals are normally passed through a multi-tap delay line in order to cause all signals received from a given point to be outputted from the delay line at the same time. This will sometimes be referred to hereinafter as "focusing" the transducer beam.
However, since the difference in distance between targets of interest and various transducer elements vary with depth, the delay line can only be focused for a given depth. This problem can be dealt with by switching taps on the delay line in an attempt to maintain the beam in focus at successively greater depths. However, such switching of taps causes transients which result in undesired artifacts in the resulting image.
DC currents flowing in the delay line as a result of differences in the average DC current at various delay line taps can also cause distortion errors in the delay line. Therefore, it is preferable that the DC current at all taps of the delay line be balanced so no net DC current flows in the delay line. Ideally, the DC current at all of the taps is substantially zero.
Currently, switching artifacts are avoided by switching only at dead times between received signals or employing parallel processing and line splicing techniques. However, this is difficult to do with high speed systems and generally results in only one or two switchings during a given scan line. This means that the beam is generally only approximately in focus and thus results in reduced image quality. It would be preferable if the switching could occur more frequently so as to permit nearly continuous focusing of the beam and thus enhanced image quality, while keeping artifacts below the level at which display occurs. For a typical ultrasonic scan display, this would be 60 db below full white or maximum signal level. Distortion caused by DC currents flowing in the delay line are not currently dealt with.